She has refused to rule out standing for the party but also described her fear of upsetting her parents who are “about the two people left... in whose eyes I can do no wrong”.

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Left to Right: Stanley, Rachel and Boris Johnson in 2014

Stanley Johnson said he thought “nothing could be more sensible” for her given her support for a second referendum on Brexit the Lib Dems have pledged.

“We don’t mastermind what our wives and family and children think. I thought it was absolutely right from her point of view,” he tells HuffPost UK. “Nobody in any way should doubt her sincerity.”

But he added there was “no conceivable way I could ever not vote Conservative because this is a matter of tribal loyalty”.

“I’m sorry to speak in these rather primitive terms but I have been, for heaven’s sake, a member of the European Parliament. It’s not quite the same thing as just joining a political party.”

Rachel Johnson’s defection excited Lib Dem leader Tim Farron into saying she would be a “marvellous” candidate for London Mayor.

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Stanley Johnson at a Remain rally just before the referendum

When asked whether he would vote Lib Dem if she were the candidate, her father paused for six seconds before saying: “Yes of course I’d vote Conservative. It would take a great deal for me not to vote Conservative.”

But he adds: “I’d be totally thrilled if she was the candidate. I’m sure she’d do a great job.”

He says: “My line has always been: Kids must do what they think. That’s the only thing that matters, isn’t it?”

Proving Boris isn’t the only one in the family prone to off-colour remarks, he adds: “My line has always been if you want to fight for Isis in Afghanistan, then off you go.” Correcting himself, he says: “No, that’s a totally flippant remark. I was just being really flippant then. I think I would put a foot down on that one.”

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Stanley Johnson at the unveiling of a life size 12ft bronze polar bear named Boris - to launch a Great British campaign to help save the polar bear

While Johnson accepts people voted for Brexit, he is determined to resist ditching the effect of certain EU directives, particularly on the issue on which he is most passionate - the environment.

His position is closer to his son’s now his daughter. He says he supports May’s “very, very sensible” handling of Brexit and backs leaving the EU provided the law on these issues isn’t diluted.

“I would say there’s a total solidarity among the male Johnsons,” he says, as another son, junior Tory minister Jo Johnson, also backed Remain but is acquiesing to Brexit after the referendum.

If Theresa May spells out in her manifesto the detail of the Brexit she is seeking, her General Election victory will have the same effect as a second referendum, he says.

“Maybe Rachel doesn’t have the confidence that there’ll be a lot of good sense in the manifesto,” he laughs.

Despite his party loyalty, Johnson speaks of Labour’s embattled leader in terms more generous than most of his own MPs.

“Mr Corbyn has some key points on workers’ rights... He has, on more than one occasion, in my hearing, mentioned the environmental issues. I have not yet heard a major Tory Party politician say: ‘We must make absolutely sure that we maintain all the good environmental stuff’.

“Andrea Leadsom has said: ‘Yes of course we’re going to have this Great Repeal Bill’. I don’t think there’s any guarantee.” He is wary of a feeling in some quarters of: “Oh ho, ho, let’s have a bonfire” of EU regulations.

Johnson is still co-chair of Environmentalists for Europe and wants the Tory manifesto to be clear on clean air, clean water, nature protection and shutting down the domestic ivory market and the use of wild animals in circuses. Much of our environmental law had its gestation in the EU, he says.

“I don’t want this Brexit business to obliterate all the other concerns,” he says. “I challenge the Government. The Brexit deal has to deliver on the environment.”

Johnson, who was an MEP from 1979 to 1984, predicts we won’t get a deal within two years and will probably leave in 2019 with a transition arrangement while we negotiate a permanent deal.

He says this is his “hunch given my experience of how long it takes to sort these things out in the EU”.

On the day he speaks to HuffPost UK, he is due to attend a debate alongside Gina Miller, the sworn enemy of hardline Brexiters whose legal challenge forced the Government to let parliament vote on beginning to leave the EU.

He calls Miller’s months-long battle through the courts a “classic example of how an individual can make a huge difference”. He believes it has made a parliamentary vote on the final Brexit deal more likely.

He also notes an “interesting little wrinkle” - the case of a retired doctor who is poised to take the Government back to court, saying a 2011 law on the EU means there must be a second referendum on Brexit.

He compared the doctor to Miller, whom he calls “totally brilliant”, but denies he would back his legal fight.